Field Effect Transistors, FET's, are known semiconductor devices useful primarily as electronic amplifiers and switches. Its structure includes a pair of spaced electrodes, referred to as the source and the drain, respectively, located on a semiconductor substrate, such as gallium arsenide treated or "doped" with impurity to provide the requisite semiconductive property, and a third electrode, the gate, located in an elevated position above and between the aforerecited electrodes. In its operation electrical voltage applied to the gate creates an electric field essentially orthogonal to the plane of the substrate containing the source and drain electrodes. The level of the voltage and, hence, the electric field, influences the level of current passing between the source and drain electrodes through the semiconductor layer when an appropriate source of electrical power is connected therebetween. In a class of those devices, those referred to as high electron mobility transistors, or simply "HEMT", the current that flows passes through the semiconducting material in an essentially two dimensional sheet like formation, abbreviated as "2DEG", that is referred to as an electron stream or gas. The details of these FET devices summarized briefly above and the techniques of fabrication are well known and are described in the technical literature to which the reader may make reference for additional details.
Others have earlier discovered that when energy in the form of light is applied to this kind of semiconductor device, the light influences the mobility of electrons and the associated electron "holes", forming the electrical current between the source and drain electrodes, within the semiconductor media. That discovery suggests that the field effect device has physical and electronic characteristics suitable for application as a photo detector, an electrical device whose resistivity to current is regulated as a function of the intensity of the applied light. An article by Chen entitled "Ultra High Speed Modulation Doped Heterostructures Field Effect Photo Detectors", C. Y. Chen et al., Applied Physics Letters 42 (12), published June 15, 1983, describes experiments in which conventional high electron mobility field effect transistors are used as photo detectors. The experiments published in the article show that for 0.83 micrometer wavelength radiation injected in a direction perpendicular to the aluminum gallium arsenide-gallium arsenide interface between the FET's gate and drain, the speed of response of the FET is limited to the transit time of photo generated electrons across the gallium arsenide region; the electron transit time between the source and drain is not a limit to the FET's speed of response as may have been the prior thinking.
Applicant has also concluded that electrons photo-generated in the gallium arsenide may be swept into the quasi metallic two dimensional electron "gas" and, as alternatively characterized, the electron "holes" are expelled from that gas, by the inherent electric field existing in the aluminum gallium arsenide-gallium arsenide heterojunction. Due to the quasi metallic nature of the two dimensional electron gas, collected photo electrons are evidenced by a virtually instantaneous increase in source to drain channel current. By quasi metallic nature, reference is made to the fact that the factor sigma, the conductivity, which is equal to the product of N, the electron density, measured in electrons per cubic centimeter; E, the electron charge, measured in Coulombs; and Mu, the electron mobility, falls within the conductivity characteristic of the metallic regime.
Given that background and the applicants observation, the present invention provides a structure for an electronic device that realizes a very high speed response to light, which is a principal object of the invention and enhances quantum mechanical conversion efficiency of light responsive devices, such as photo detectors, an additional object of the invention. A further object is to provide a reduced electron drift time in a high electron mobility field effect type transistor photo detector. A still further object of the invention is to provide a photo detector utilizing semi-conductor material, such as gallium arsenide, having high optical responsivity, low noise, very high speed response, in the form of a monolithic structure capable of being manufactured by ordinary transistor fabrication techniques and which may be combined with other semiconductor devices in a monolithic or integral structure.